1. Field of the Invention
The invention relates to transducers for use in determining the velocity of an object and, more particularly, to a fiberoptic velocity transducer utilized in the measurement of rotational speed of an aircraft braking system component.
2. Description of the Related Art
Since the incorporation of rotating members into machinery, there has been a need to observe and control the velocity of the member in order to meet system efficiency and responding requirements. Such requirements have become increasingly stringent for high performance vehicular systems, particularly those connected with braking system of aircraft and other high speed vehicles. For such systems, it is critical to know the rate at which the rotational speed of the wheels is decreasing in response to application of the braking system.
One means utilized in the past to determine the rotational speed of an object is a form of electromechanic tachometer. The electromechanic tachometers typically functioned to generate signals proportional to the speed of the rotating members but were limited by, among other things, electronic noise existing in the environment in which they were used. While some improvements to the early electromechanic tachometers were helpful in reducing the signal noise and improving the resolution of the tachometer, the electromechanic instruments still left much to be desired as a primary instrument for measuring velocity of a braking system component such as the brake disk or wheel.
Later developments in rotational speed sensors included thee use of light pulse signals, where the number of pulses observed by an electronic control unit correlated to the speed of the disk. Due to their EMI insensitivity, light weight and high resolution systems utilizing light to determine rotational speed were particularly desirable. These systems, however, had a nonhermetically sealed optical cavity which can be detrimental in applications (such as aircraft wheelspeed sensing) that are required to go through significant pressure changes in a dirty environment.
Further improvements included velocity transducers utilizing magneto-optic material configured to cooperate with fiberoptic light energy which were incorporated into systems for determining rotational speed. Such systems not only embodied improved means for transmitting light pulses to electronic controllers, but were able to take advantage of Faraday rotation properties of the magneto-optic material to hermetically seal the optical cavity. Generally speaking, when subjected to a magnetic field, a particular magnetic flux density is created in the magneto-optic material which functions to cause polarized light passing through the material to undergo a Faraday rotation. Coupling then occurs between the electric field vectors of photons comprising the polarized light energy passing through the magneto-optic material and electric field vectors of the magnetically aligned atoms of the magneto-optic material. When such coupling occurs, the polarization vectors of the light energy are rotated, with the amount of Faraday rotation being determined by the magnetic flux density of the magneto-optic material. By configuring a rotating member with means to affect the magnetic flux density of the magneto-optic material to varying degrees based upon its speed and by noting the relationship between the resulting amount of rotation of the polarization vectors and the speed of the member, it was possible to determine the speed of the rotating member by monitoring the resulting polarization vectors. In this configuration, the light is modulated by the varying magnetic flux, and since an encoder disc is not necessary, the optical cavity that contains elements through which the light propagates can be hermetically sealed.
In conventional applications of the velocity transducers employing magneto-optic material, the magneto-optic material is used to facilitate the development of light energy signals used to determine the speed of a rotating member. The magneto-optic material was carefully chosen so that it caused only a narrow bandwidth of wavelengths of polarized light to rotate. Accordingly, a single light source with a broad spectrum of wavelengths was conventionally propagated through the magneto-optic material, the magneto-optic material functioning to rotate the light energy having a particular range of wavelengths within the bandwidth and leave unaffected the light energy with wavelengths outside the bandwidth. In this way, the unaffected light energy was used as a reference in the calculation of rotational speed, or as a reference for an integrity check, and the rotated light energy as an indication of rotational speed. In the alternative, two light sources were simultaneously utilized and propagated through the magneto-optic material, one having a wavelength outside the bandwidth, and thus utilized as a reference, and the other being rotated by the magneto-optic material and used as an indicator of rotational speed. In either case, the conventional velocity transducer typically comprised a mirror operating to reflect the light waves, irrespective of their wavelengths, back through the magneto-optic material.
These conventional velocity transducers incorporating magneto-optic material were limited, however, since they required relatively expensive parts and materials. The magneto-optic material was expensive since, in order to facilitate the development of a light energy reference, it was required to effect only a narrow bandwidth of wavelengths of light. In addition, the light source light detectors and optical fibers were expensive since the light wavelength which is not affected by the magneto-optic material is normally a longer wavelength than the ones used for standard communication fiber-optic components.
To circumvent or overcome the problems and limitations associated with prior art velocity transducers, a velocity transducer comprised of relatively inexpensive parts and materials and that generates signals which accurately correspond to the rotational speed of a member is highly desirable. The present invention fulfills this need.